1. Field of the Invention
The present invention relates to a signal processing apparatus and method in which a bit stream supplied as an input is decoded to an image signal and the image signal is provided as a bit stream again.
2. Description of Related Art
For video signal compression, MPEG (moving picture coding experts group) and MPEG-2 have been used more widely in combination with a redundancy reduction by an orthogonal transform such as motion compensation (MC), discrete cosine transform (DCT) or the like.
An audio data not compressed is compressed according to such an MPEG to be a coded image such as an intra-picture coded image (I picture), forward -predictive coded image (P picture) or bi-directional predictive coded image (B picture) for storage into a cumulative recording medium such as magneto-optic disc or the like or for transmission over a communications network.
In this MPEG technique, a bit stream is decoded to be a picture signal which will be transformed into a bit stream again. For transform from a movie film to a video film, four frames are subjected to a so-called 2:3 pulldown processing to produce five frames in order to transform 24 frames/sec on the movie film to 30 frames/sec on a TV/video film.
FIG. 1 illustrates, in a schematic block form, a conventional signal processor in which a bit stream of a moving picture is decoded and the decoded moving picture is coded according to the MPEG-2 Video Compression Standard to output the bit stream.
As shown, the conventional signal processor includes a decoder 101 to decode an external input bit stream S26 to a picture signal, a post-processing circuit 102 to post-process the decoded picture signal, a pre-processing circuit 103 to pre-process the picture signal for coding, and an encoder 104 to code the picture signal from the pre-processing circuit 103.
The decoder 101 decodes the input bit stream S26 to provide a decoded picture S20. The decoder 101 also decodes a display control parameter S23 from the input bit stream S26 and supplies it to the post-processing circuit 102. The display control parameter S23 prescribed in the MPEG-2 Video Compression Standard is horizontal size, vertical_size, frame rate, display_horizontal_size, display_vertical_size, aspect_ratio_information, progressive_frame, repeat_first_field, top_field_first, frame_centre_horizontal_offset, frame_centre_vertical_offset.
The post-processing circuit 102 processes the decoded picture S20 based on the display control parameter S23 to provide a post-processed image S21. More particularly, the post-processing circuit 102 effects 2:3 pulldown based on top_field_first, repeat_first_field, upsampling of chroma signal from 4:2:0 format to 4:2:2 format based on progressive_frame, and also clipping of a rectangular window from an image and switching to sampling in a display frame size, namely, upsampling or downsampling, based on horizontal_size, vertical_size, display_horizontal_size, display_vertical_size, aspect_ratio_information, frame_centre_horizontal_offset, frame_centre_vertical_offset.
The 4:2:0 format is a component format for coding of video signal, adopted in MP@ML (main profile/main level) of MPEG-2. The FIGS. 4, 2 and 0 indicate a sampling frequency ratio, that is, resolution ratio, between a brightness (Y) signal included in the horizontal line (scanning line) of a video signal and two chrominance signals (Cb and Cr). The figures in the 4:2:2 format also indicate such ratios.
The post-processed image S21 is supplied to the pre-processing circuit 103 in which the post-processed image S21 is subjected to inverse 3:2 pulldown, downsampling of chroma signal from the 4:2:2 format to 4:2:0 format, etc. to provide a pre-processed signal S22. Also the pre-processing circuit 103 provides a display control parameter S24 indicative of the content of the inverse processing from the pre-processed image S22 to post-processed image S21.
The encoder 104 codes the pre-processed image S22 and also codes the display control parameter S24 of the pre-processed image S22 as a header information of a bit stream of the coded pre-processed image S22 to provide a bit stream S27.
It is well known that for coding an image with a high quality when decoding an MPEG video bit stream and re-coding the decoded image according to MPEG for providing an bit stream, rendering a picture_coding_type being a coding parameter S25 for coding the image same as that of the original bit stream is effective. For this purpose, the coding parameter S25 is supplied from the decoder 101 to the encoder 104. The information carried by the coding parameter S25 is effected when the decoded image S20 corresponds one-to-one to the pre-processed image S22.
In some cases, however, the coding parameter S25 cannot effectively be used since the one-to-one correspondence between the decoded image S20 and pre-processed image S22 cannot be assured.
As the causes of the decoded image S20 and pre-processed image S22 not corresponding one-to-one to each other, the inverse 2:3 pulldown of the post-processed image S21 by the pre-processing circuit 103 is not any completely inverse one, that the post-processing circuit 102 has switched to sampling in the display frame size, etc.
Also, repeated upsampling of chroma signal in the post-processing circuit 102 and downsampling of chroma signal in the pre-processing circuit 103 will also cause the chroma signal to be deteriorated.
The external bit streams S26 include ones based on different signal standards. Images S20 decoded from such external bit streams S26 in the decoder 101 are different in number of pixels per picture and number of pictures per unit time. For transmission of such images, interfaces meeting the different signal standards have to be used and also hardware and software have to be used which meet such standards.
It is therefore an object of the present invention to overcome the above drawbacks of the prior art by providing a signal processing apparatus and method in which coding parameters can be effectively used to code, with a high quality, an image decoded from a bit stream coded from an image signal.
It is another object of the present invention to provide a signal processing apparatus and method, signal decoder and signal encoder in which for transmission of an image decoded from a bit stream coded from an image signal, an existing interface can be used even for images prepared according to different standards and coding parameters can be effectively used to code, with a high quality, an image decoded from a bit stream coded from an image signal.
The above object can be attained by providing a signal processing apparatus comprising, according to the present invention:
means for decoding a bit stream coded from an image signal to provide an image signal and a coding parameter;
a first switching means for selecting a destination to which the image signal from the decoding means is to be sent;
means for post-processing the image signal from a selectable terminal of the first switching means;
means for pre-processing the image signal;
a second switching means for selecting either the image signal from the pre-processing means or the image signal from the first switching means; and
means for coding the image signal from the second switching means and the coding parameter from the decoding means.
The above object can also be attained by providing a signal processing method in which a signal processing apparatus including means for decoding a bit stream coded from an image signal to provide an image signal and a coding parameter; a first switching means for selecting a destination to which the image signal from the decoding means is to be sent; means for post-processing the image signal from a selectable terminal of the first switching means; means for pre-processing the image signal; a second switching means for selecting either the image signal from the pre-processing means or the image signal from the first switching means; and means for coding the image signal from the second switching means and the coding parameter from the decoding means, comprising, according to the present invention, the steps of:
decoding a bit stream to an image signal by the decoding means; and
coding the image signal from the decoding step by the coding means.
The above object can also be attained by providing a signal processing apparatus and method in which, according to the present invention, a bit stream coded from an image signal is decoded to provide an image signal and a coding parameter; a dummy signal is inserted to convert the decoded image signal to a predetermined format, to thereby provide the image signal in the predetermined format; the image signal in the predetermined format including the dummy signal is transmitted; the dummy signal is removed from the transmitted image signal; and the image signal from which the dummy signal has been removed and the coding parameter are coded together.
The above object can also be attained by providing a signal decoder comprising, according to the present invention:
means for decoding a bit stream coded from an image signal to provide an image signal and a coding parameter; and
means for inserting a dummy signal to convert the image signal from the decoding means to a predetermined format, to thereby provide the image signal in the predetermined format.
The above object can also be attained by providing a signal encoder comprising, according to the present invention:
means for receiving an image signal in which a dummy signal is inserted to convert an image signal decoded from a bit stream coded from an image signal to a predetermined format and removing the dummy signal from the image signal in the predetermined format including the dummy signal; and
means for coding the image signal from which the dummy signal has been removed and a coding parameter obtained during the decoding to provide a bit stream.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.